Monitoring or tracking eye movements and gaze points can be used in many different contexts. Information about the position at which a person is looking can be used for analyzing the behavior and consciousness of the person. It can be used both for evaluating the object at which the person is looking and for evaluating the respective person. The fields of use are many and thereamong studies on the usability of software and different types of interfaces, evaluations of home pages, advertising and advertisements, means for educating pilots in simulator surroundings, research in psychology, behavior science and human perception and diagnosis of various types of visual faults and illnesses can be mentioned.
In addition to these applications also interactive applications exist which employ information about the place at which a person is looking in order to respond or react in different ways. A physically disabled person can for example interact with a computer by the procedure that those objects on the monitor of the computer at which he is looking are activated. In an arcade game the adventure experience of the game can be very much enhanced by the procedure that those objects at which a person is looking are made to be centered in the image or by the procedure that the person is allowed to direct a weapon using her/his eyes. In the same way the advertising or display for Christmas in a shop window can react to the fact that a person is looking at it. Interfaces for for example a computer could utilize continuous information about the position at which a user is looking to be better capable of displaying the object in which a user is interested and of adapting in an intelligent way to different behaviors of the user. A motorcar that receives information about the position at which the driver is looking could issue an alarm in the case where the driver is tired, distracted or intoxicated. Military applications for directing weapons or for steering vehicles using eye movements have also been developed. These are only a small number of the fields of use that can be of interest for an installation that can detect and track eyes and gaze angles.
Many different technologies exist that can be used for tracking an eye and the gaze angle thereof. A first technology is to use a photosensor and a light source, the light of which is reflected from the cornea and from the lens of an eye and thereby causes four different reflections. The reflection thereof having the strongest light intensity is the outer cornea reflection or glint, also called the first Purkinje reflection. Thereafter the second, the third and fourth Purkinje reflections follow which correspond to reflections in the inner surface of the cornea, the outer surface of the lens and the inner surface of the lens. The first and fourth Purkinje-reflections are located in the same focal plane and the relationship of the positions of these two reflections varies with rotation of the eye.
A second technology for tracking an eye and the gaze angle thereof is to detect, using some type of photosensor, the boundary line between the white or sclera and the iris of an eye. A third technology is to detect the face of person using a camera and to indirectly deduce, by observing how the eyes move in relation to the face, the direction in which the person is looking. A fourth technology is to use that the cornea and the retina have different electrical potentials for measuring rotation of an eye. A fifth technology is to use a contact lens fitted with suitable devices to be capable of measuring how the contact lens and hence also the eye rotates. A sixth technology is to measure, using a photosensor, the ellipticity of the pupil or of the iris when the eye rotates. A seventh technology is to use a diode and two or more optical transistors mounted in a pair of spectacles. When the light from the diode is reflected from the cornea and then hits an optical transistor the signals provided by the transistor will vary in accordance with the travel distance of the light from the diode via the cornea and to the transistor. An eight technology is to use a photosensor to detect the reflection of a light source from the cornea and then to observe how the position thereof is related to the center of the pupil or iris. The pupil can in this case be made more easily observable by using illumination devices mounted close to the photosensor illuminating the retina of the eye and thereby causing a bright eye effect or by illuminating the eye so that the iris is made bright whereas the pupil remains dark.
In U.S. Pat. No. 5,861,940 a system for detecting eyes and for tracking gaze angles is disclosed. The system includes light sources comprising an IR-laser 2 that is provided with a suitable optical system and issues a convergent light beam with a focus in front of the eye and two IR-lasers or IR-diodes 9, 10 that alternatingly emit divergent light beams. By detecting the reflection from the retina using a position sensitive detector information about the position of the eye is obtained. By detecting the reflections of a plurality of light sources from the cornea the distance of the eye from the video camera is determined. The latter two IR-lasers are placed at the bottom edge of a computer monitor 1. Other systems are disclosed in U.S. Pat. Nos. 6,079,829, 6,152,563 and 6,246,779.
In automatic systems that employ a video camera it can be difficult to determine the positions in a captured picture, where reflections of light sources from the eyes of a user are located. This depends among other things on the fact the user can move her/his head, displace it laterally or to places at a smaller or larger distance of the camera. The automatic system can contain complex image processing and image recognition routines for determining those areas where images of eyes are located. Hence there is a need for methods for facilitating such determinations of areas in a captured picture in which reflections and images of eyes are located. Furthermore, in determining the point of regard on a surface such as a monitor, always the distance between the eyes of the user and the monitor must be determined and thus efficient methods for such a determination are required.